[0001] This invention relates to a method of, and apparatus for, processing liquid metal,
e.g. steel.
[0002] For many metallurgical processes it is necessary or advantageous to deliver liquid
metal from one vessel into a succeeding vessel in the processing route, eg. a metal
forming medium, in a controlled, low turbulence, low velocity mode. Additionally,
it is advantageous for the liquid metal to be maintained in a "clean" condition for
delivery to such a forming medium. Such a forming medium might for example be a casting
mould, continuous casting mould, ingot mould or any container one of whose functions
is the extraction of heat from the liquid metal to produce solidification. The advantages
- operational, metallurgical and economic - of casting or forming from a stock of
clean liquid metal delivered under controllable physical conditions are indeed well
known.
[0003] Numerous systems are known by which liquid metal is dispensed into a feed container
and thence via a transport medium into such a metal forming medium. The feed container,
which may be any container capable of supporting a quantity of liquid metal sufficient
to form a reservoir of the feed metal, will not normally ensure a controlled rate
of flow, and the flow of liquid metal associated with high degrees of turbulence and
high velocities into a metal forming medium is detrimental to the operation of the
metal forming medium; further it is often advantageous to be able to control the rate
of formation of, and the position of, the solidifying product such that attenuation
of rates of growth or remelting of the product is discouraged. Moreover, it is usually
the case that the liquid metal contained within the metal forming medium will be covered
by a liquid non-metallic covering, which may for example be a mould lubricant, and
the possibility of entrainment of such non-metallic substances into the liquid metal
body away from the liquid metal surface, which is enhanced by high velocities and
high turbulence, is detrimental to the product quality.
[0004] Additionally, it is most desirable to prevent the flow of non-metallic contaminants
into the metal forming medium or their deposition on the walls of the transport medium
which, e.g., may be a hollow tube fitted with a flow control device (for example a
stopper and nozzle arrangement) attached to a submerged entry shroud, and it is a
characteristic of non-metallic contaminants that they are deposited in regions where
conditions of high metallostatic head, high turbulence and high velocity, apply. The
areas where these contaminants are greatly detrimental to process, metallurgical and
economic operation lie in the quality of the solidified product and in the premature
withdrawal from service of otherwise servicable items of equipment due to non-metallic
contaminant deposition.
[0005] It is the object of this invention to mitigate these problems.
[0006] According to one aspect of this invention there is provided a vessel having a first
chamber open to atmospheric pressure for receiving liquid metal and a second chamber
from which the metal is dispensed interconnected with the first chamber the second
chamber being sealed and coupled to means for reducing the pressure therein to sub-atmospheric
whereby to create a higher level of metal in said second than said first chamber.
[0007] The invention effects a reduction in the metallostatic head within the second chamber
such that the metal velocity at the outlet from same is greatly reduced.
[0008] To achieve the principal object of this invention any pressure reducing medium eg.
a barrier having one or more orifices may be employed but, preferably, the first and
second chambers are interconnected via a filter.
[0009] The difference in the pressures imposed on the liquid metal within the two chambers
is preferably maintained at as high a level as is practicable since, in general, the
'cleaning' efficiency of the filter(s) increases with the energy dissipated by the
liquid metal as it passes through. This invention thus allows the use of higher efficiency
filters than has been possible hitherto.
[0010] The filter may be constructed from a refractory material having an affinity for the
particular contaminants held in suspension within the liquid metal. It may be of monolithic
or composite construction and the pores (which may for example be composed of slots,
cylindrical holes or any open cell form) must be sufficiently small to allow the pressure
gradients to be maintained at the desired flow rates and yet sufficiently large to
withstand a degree of contaminant deposition without blocking the material.
[0011] Because the metal velocity at the outlet from the second vessel is reduced, flow
control devices sited at the outlet can operate with greatly increased flow exit areas,
lowering the rates of contaminant deposition on the surfaces of these devices which
otherwise restrict the metal flow rate and promote premature failure. In particular
the outlet from the second chamber may be constituted by e.g. a slide gate valve or
the like and this may surmount a hollow shroud through which the metal is transported
into, say, a continuous casting mould or an ingot mould or the like.
[0012] As mentioned, with the consequent 'delivery' velocity of the metal into the forming
medium, and its turbulence, reduced the tendency hitherto for e.g. non-metallic mould
coverings to become entrained within this metal flow into the mould, adversely affecting
the product quality in thus mitigated.
[0013] Other advantages and benefits arising from this invention will be more apparent from
the following descriptions where, in order that the invention may be fully understood,
one embodiment thereof will now be described, by way of example, with reference to
the accompanying drawings, in which:-
Figures 1 and 2 schematically illustrate two forms of apparatus for performing the
method of this invention.
[0014] In the following description, the term "valve" relates to any form of flow control
device.
[0015] Referring now to the drawings, Figure 1 shows the operation of the apparatus during
the preheat phase. The apparatus consists of a first chamber connected to the open
atmosphere and a second chamber 2 interconnected thereto via an apertured wall. The
chamber 2 is enclosed and connected to two pressure reduction devices 4 and 6. Pressure
reduction device 4 is a low differential pressure, high volume pump or ejector connected
to chamber 2 via a valve 5 and pressure reduction device 6, a high differential low
volume pump or ejector, is connected to chamber 2 via a valve 7. An outlet 8 is connected
to a metal forming medium 9 via a valve 10.
[0016] A burner 11 introduces hot gases into chamber 1, the temperature of the gases preferably
exceeding the liquidus temperature of the liquid metal to be processed.
[0017] The purpose of this preheat phase is to raise the temperature of the wall 3 to a
level whereby liquid metal can be introduced into chamber 1 and flow into 2 without
the liquid metal solidifying at this juncture. To achieve this purpose, valve 5 is
open, valves 7 and are closed and pump 4 is used to withdraw gases from within the
chamber 2. The pressure within 2 will fall and the resulting pressure difference between
the chambers 1 and 2 will cause hot gases produced by 11 to flow from 1 to 2 through
the apertured wall. In this manner heat will be transferred from the hot gases to
this wall thereby raising its temperature.
[0018] Figure 2 shows the apparatus as used during the production phase.
[0019] In particular liquid metal flows under the influence of gravity from a container
12, e.g. a ladle, through a pour tube 13 into chamber 1. A flow control device 14,
which may for example be a stopper and nozzle arrangement, is used to control the
level of liquid metal in chamber 1. Valve 5 is closed and valves 7 and 10 are open.
The pressure within the gas space 15 above the liquid level chamber 2 is thus reduced
by the pump 6 and liquid metal flows from chamber 1 into 2 through the apertured wall
under the influence of the substantial difference in pressure across this item. A
liquid level monitor 16 is used to monitor the liquid level in chamber 2 and a pressure
measuring device 17 is used to monitor the pressure within the gas space above the
liquid metal in chamber 2 and may be used to adjust the operation of the pump 6 or
the position of valve 7 so as to control the monitored pressure level.
[0020] Liquid metal flows from chamber 2 through the open valve into the forming medium
9. The metallostatic pressure at the outlet 8 and valve 10 resulting from the column
of liquid metal above them is maintained at substantially lower levels than would
have been the case if chamber 2 had not been enclosed. The effect of the sub-atmospheric
pressure maintained in 15 partially opposes the metallostatic force at the inlet to
valve 10 and thereby reduces the pressure difference across this valve. The resulting
lower pressure difference across this valve compared with the case where the gas space
15 is not maintained at sub-atmospheric pressure, allows the design of valve 10 to
be such as to present a larger area for flow of liquid metal at similar flow rates.
In turn, the use of the larger areas for flow results in substantially increased distances
between the flow control surfaces of valve 10 and also produces lower liquid metal
velocities. Now since non-metallic contaminant deposition tends to increase with velocity,
both the lower velocities and the larger inter-facial distances associated with the
flow control surfaces substantially decrease the possibility of contaminant deposition
and subsequent premature blockage in the regions of outlet 8 and valve 10. In turn,
the low exit velocities of liquid metal from valve 10 will result in low delivery
velocities into the forming medium 9.
[0021] During the production phase there will be occasions when the level of liquid metal
within the ladle 12 falls to where production must be suspended and another ladle
employed to replace 12. During this interval, there will be no flow of liquid metal
into chamber Normally, the liquid metal in chamber 1 will be covered by a layer of
non-metallic material. Hitherto, where liquid metal flows from a ladle such as 12
into an open container, for example a tundish, and thence into a metal forming medium,
it has been found that non-metallic contaminants from this layer may be entrained
within the liquid metal and be fed into the metal forming medium, thus contaminating
the product. With the present invention however such contamination is eliminated by
a combination of lower exit flow rates of liquid metal through the valve 10, by adjusting
its setting, and maintaining a level of liquid metal in chamber 2 by using chamber
1 as a stock feed reservoir. Where the duration of such an interval between ladle
changes is long, the chamber 2 can also serve as a stock feed reservoir once the stock
of liquid metal in chamber has become exhausted. This is made possible because the
pressure in 15 is controllable thus allowing the metal level in 2 to be raised and
lowered independently of the metal level in chamber 1.
[0022] During the production process, it may be desirable to introduce substances, e.g.
chemical reactants, which enhance the properties of the formed product. It is advantageous
to add some substances at as late a stage in the production process as possible since
this eliminates the possibilities of some problems, for example segregation of alloying
elements. Additionally they may be employed to reduce the level of superheat which
might otherwise be required. Argon (injected at 18) may be used as a carrier gas for
the introduction of these substances, the gas rising to the surface of the liquid
and being exhausted from 15 by the pump 6.
[0023] Preferably, the apertured wall 3 is constituted by a cleaning or filtering device.
In particular, the operation of the filter is such that non metallic contaminants
suspended in the liquid metal are caused to be deposited on the surface of the material
from which it is constructed, eg. a refractory or ceramic. The high pressure drop
across this filter and small pore size causes the metal to accelerate as it passes
through and the highly turbulent nature associated with this region of the flow produces
a very highly efficient cleaning process.
[0024] Non-metallic contaminants will of course be deposited on the surface of the filter
and as this process continues the pressure difference across this item (3) required
to maintain a constant flow of liquid metal from chamber 1 to chamber 2 will increase
otherwise the flow of liquid metal will decrease. Liquid metal flow rates can be maintained
by increasing the level of liquid metal in chamber 1 by adjusting the flow rate of
metal from the ladle 12 such that the metallostatic pressure at the base of chamber
1 increases to compensate for higher required pressure gradients across the filter
and indications that such action is required will be obtained by identifying a reduction
in the level of product in the forming medium 9.
[0025] Although the invention has been described with reference to the particular embodiments
illustrated, it is to be understood that various modifications may readily be made
without departing from the scope of this invention. For example, the particular configuration
of the apparatus described in the drawings and the dispositon of the control and/or
monitoring equipment may well be different from that shown.
1. A vessel having a first chamber open to atmospheric pressure for receiving liquid
metal and a second chamber from which the metal is dispensed interconnected with the
first chamber, characterised by the second chamber 2 being sealed and coupled to means
4-7 for reducing the pressure therein to sub-atmospheric whereby to create a higher
level of metal in said second than said first chamber.
2. A vessel according to claim 1, characterised in that the first and second chambers
are interconnected via an apertured wall
3. A vessel according to claim 1, characterised in that the first and second chambers
are interconnected via a wall in which a filter is sited.
4. A vessel according to claim 3, characterised in that the filter is constructed
from a porous refractory material.
5. A vessel according to any one of claims 1 to 4, characterised in that the pressure
reducing means comprises a low differential pressure, high volume pump 4 and a high
differential pressure, low volume pump 6, each selectively operable and connected
to the said second chamber via separate valves 5,7.
6. A vessel according to any one of claims 1 to 5, characterised in that reactants
or alloying additions are introduced into the liquid metal in the second chamber.
7. A vessel according to any one of claims 1 to 6, characterised in that the liquid
metal is dispensed from the second chamber via a slide gate valve.
8. A method of processing liquid metal in a vessel according to any one of the preceding
claims, characterised in that the first chamber is pre-heated before the introduction
of liquid metal, the pressure within the second chamber is reduced whilst the discharge
of said metal therefrom is arrested to enable a head of the metal to be built up in
said second chamber, and the pressure is thereafter controlled during subsequent discharge
to maintain a desired metallostatic pressure at the point of discharge.